1. Field of the Invention
The present invention relates to webbing for a seat belt, which is woven in a belt like configuration by making warp yarns cross with weft yarns.
More particularly, the invention relates to a technique of obtaining a thin and light webbing having a high strength per unit numbers of warp yarns and maintaining abrasion resistance, which is woven by using, as warp yarns, polyester multifilament yarns each made by very-strong, fine single filament yarn which are not typically used in a conventional seat belt and are highly twisted very much with a twisting condition, unlike in the conventional seat belt.
2. Description of Related Arts
Generally, a webbing for a seat belt has specifications assuring characteristics such as specified tensile strength, elongation, and durability or the like, and it mainly uses, as warp yarns, multifilament yarns made of polyester or polyamide filament yarns or fibers and, as weft yarns, multifilament yarns or monofilament yarns made of polyester or polyamide filament yarns of fibers, and is woven into a belt like narrow fabric by making the warp and weft yarns intercrossed with each other on a loom for a narrow fabric.
Conventionally, most of webbings are woven into a belt like narrow fabric on a power loom for a narrow fabric by making warp and weft yarns intercrossed with each other by reciprocating a shuttle for weft yarns along a shedding portion provided within the warp yarns and formed by an upper warp yarn group and a lower warp yarn of the total warp yarn group and separated from each other, on the loom.
During the weaving operation of the power loom for a narrow fabric, due to abrasion to the warp yarn group by the shuttle for weft yarns which passes through the shedding portion formed by the upper warp yarn group and a lower warp yarn of the warp yarns on the loom, damage such as a breakage in a filament often occurs on the warp yarn group.
To prevent the breakage in a filament of the warp yarn group, conventionally, a multifilament yarn made of polyamide, which has a single yarn fineness of about 8 deniers and a strength thereof being about 8 g per denier and is twisted up to about 80 turns/m, had been generally used as a warp yarn in a webbing.
After that, a weaving process for making webbing by utilizing a needle loom for a narrow fabric has become popular. In the weaving, by reciprocating a needle for weft yarns along the shedding portion formed by separated upper warp yarn group and lower warp yarn group on the loom, the warp yarns and the weft yarns are intercrossed with each other so as to farm a belt like configuration.
Abrasion to the warp yarn group caused by the needle for weft yarns, which passes through within the shedding portion formed by the separated upper warp yarn group and lower warp yarn group, on the loom, during the weaving operation by the needle loom for narrow belt like fabric, is much smaller than that to the warp yarns caused by the shuttle for weft yarns used in a power loom.
Therefore, without maintaining binding characteristics of yarns by giving certain numbers of twist in the warp yarns, the damages such as the breakage in a filament does not occur so much on the warp yarn group. Consequently, the tendency of reducing the cost of a webbing by eliminating the coat of the twisting process by skipping the twisting process of the warp yarns, which is conventionally performed, became stronger.
Further, along with the above-mentioned tendency of not performing the twisting process, a new method for sec wing abrasion resistance of a webbing by performing a surface finishing process using a resin, oil, or the like on the webbing woven by using, as warp yarns, multifilament yarns of which single yarn fineness is large, was created and this technique has been often used.
As a result, it became common to use, as a warp yarn, a multifilament yarn made of polyamide or polyester, almost without being twisted, and having a single yarn fineness of about 11 deniers and a strength of about 8.5 g per denier.
Note that, recently, a polyester fiber is used in most of webbings and thus it becomes a mainstream in manufacturing a webbing, to weave a webbing on a needle loom for a narrow fabric by using, as a warp yarn, a polyester multifilament yarn having a single yarn fineness of about 11 deniers and a strength of about 9 g per denier.
Further, recently, with advance in the technique of making polyester yarns in which the yarn can maintain conventional yarn strength although a single yarn fineness thereof is large, and in which filaments are seldomly broken, in order to omit a surface finishing process to the yarns for maintaining abrasion resistance of the webbing, there is a tendency to further enlarge the mingle yarn fineness of a fiber used for a warp yarn in the webbing.
It is recommended to use, as a warp yarn in a webbing, a polyester multifilament yarn having a single yarn fineness of about 14 deniers and a yarn strength of about 9 g per denier with an almost no-twist.
As in the transition of the conventional technique, the needle loom for a narrow fabric has come to be generally used for weaving of webbing. As the yarn-spinning technique advances and as a fiber manufacturer has come to be able to supply a polyester multifilament which is hardly broken and has a large single yarn fineness, that was difficult to be achieved in the past, the webbing manufacturing technique has been shifted to the direction of omitting the process of twisting the warp yarn in a webbing to reduce the cost of the webbing and of increasing the fineness of a single yarn of a polyester filaments or fibers used for a warp yarn to secure abrasion resistance by omitting a surface finishing treatment process.
On the other hand, the higher the drawing ratio of a filament or fiber becomes, the higher the strength becomes. On the contrary, occurrence of breakages in a filament increases.
As the fineness of a single yarn is increased, the drawing process in a yarn spinning process becomes difficult. It is that the yarn speed of yarn spinning process decreases to keep a predetermined quality.
Even by utilizing the present yarn-spinning technique which is the to be highly advanced, it is very difficult to further improve the strength of the fiber by increasing the drawing ratio or to further reduce the cost of a source yarn by increasing the yarn-spinning speed without increasing the occurrence of failures, for example, generation of yarn break or the like in the filament of the polyester multifilament yarn or fiber having an increased single yarn fineness which is being often used recently as the warp yarn in a webbing.
On the other hand, in a weaving process, a polyester multifilament yarns having a large single yarn fineness and almost without being twisted, is used for the warp yarns in a webbing.
Therefore, the drawback caused by yarn breakages in the filaments becomes a drawback of a webbing in that a cut cad portion of a single filament or fiber having a relatively large fineness is projected from the surface of the woven webbing, and is apparent as a failure in the appearance of the webbing.
From the viewpoint of suppressing a failure in the webbing, frequent occurrence of the failure of yarn breakages in the filament in the yarn-making process cannot be accepted, while, it is difficult to pursue reduction in the cost of the source yarn by increasing the strength of the filament or the fiber or by increasing the yarn-making speed.
Therefore in the direction of the recent webbing weaving technique of increasing the fineness of a single yarn as well as of omitting the process of twisting the polyester yarns used for the warp yarns of a webbing, it is difficult to reduce the cost of the source yarn by improving the speed of spinning a yarn in the yaw-spinning process or to reduce the cost of the source yarn by improving the strength of the source yarn by increasing the drawing ratio. The techniques have reached the highest limit and the next task is to develop a novel technique to look for a remarkable breakthrough on this matter.
It is therefore an object of the invention to overcome the above-mentioned problems in the past and to provide a thinner, lighter, and cheaper webbing having a capability of reducing a certain number of warp yarns therein, as well as having a higher strength per unit number of warp yarns and a higher tensile strength, which enables polyester multifilament yarns having very high levels of yarn strength and comprising a plurality of single polyester filaments having very fine fineness, to be used as warp yarns of the webbing without deteriorating abrasion resistance of the webbing.
To achieve the object of the present invention, the invention basically employs technical configurations as described below.
As a first aspect of the present invention, there is provided a webbing for a seat belt, which comprises warp yarns and weft yarns, and wherein polyester multifilament yarns having a yarn strength of 10 g/denier or more, and each comprising a plurality of polyester single filaments, each having a single filament fineness of 7 denier or less in average and further each one of the polyester multifilament yarns being twisted with a twist coefficient xcex1 of 4000 or more, the twist coefficient xcex1 being obtained by the following equation (1), are used for main ground warp yarns of the webbing, and further wherein the webbing being woven by the warp yarns and the waft yarns and utilizing a weft yarn density which satisfies a relationship between the weft yarn density and a fineness of the weft yarn as defined by the following equation (2),
xcex1=Txc3x97(D1)1/2xe2x80x83xe2x80x83(1) 
(where T denotes a number of twist t/m, and D1 indicates a denier of multifilament yarn as used for the warp yarns); and
As a second aspect of the present invention, there is provided a method of manufacturing webbing for a seat belt, which comprising the steps of:
arranging a plurality of polyester multifilament yarns, each comprising a plurality of polyester multifilament yarns having a yarn strength of 10 g/denier or more, and each comprising a plurality of polyester single filaments each having a single filament fineness of 7 denier or less in average and further each one of the polyester multifilament yarns being twisted with a twist coefficient xcex1 of 4000 or more, the twist coefficient xcex1 being defined by the above-mentioned equation 1, as at least one part of the ground warp yarns of the webbing;
supplying the polyester multifilament yarns to a weaving process with twisting same simultaneously; and
weaving the webbing with the polyester multifilament yarns as the warp yarns and predetermined yarns as the weft yarns.
Since the webbing for a seat belt and the method of manufacturing a webbing for seat belt in the present invention employs the above-described technical configuration, a thinner, lighter, and cheaper webbing having a capability of reducing a certain number of warp yarns therein, as well as having a higher strength per unit number of warp yarns and a higher tensile strength, which enables polyester multifilament yarns having very high levels of yarn strength and comprising a plurality of single polyester filaments having, very fine fineness to be used as warp yarns of the webbing without deteriorating abrasion resistance of the webbing can be easily obtained.
In the present invention, a polyester multifilament having a single-yarn fineness being remarkably much smaller than that used for a warp yarn recently and generally, which contains therein a plurality of polyester yarns having relatively high level of yarn fineness, and specifically, a polyester multifilament yarn containing therein a plurality of polyester filament yarn each having a single yarn fineness of 7 deniers or less and the yarn having a much higher yarn strength, specifically, a strength of 10 g/denier or higher is used as a warp yarn in a webbing.
The yarn having a smaller single-yarn fineness can be produced in a filament spinning process with a drawing ratio higher than that as used in the conventional yarn spinning process and thus the polyester filament yarns having a yarn strength being much higher than that of the conventional one, can be used as a warp yarn.
And further, the yarn can be produced at a higher speed, and the polyester filament yarn, by which cost can be largely reduced, can be used as a warp yarn of webbing. Therefore, the strength of a webbing can be improved by increasing the strength of the warp yarn of the webbing or the cost of the webbing can be reduced by reducing the amount of warp yarns as used in a webbing and by reducing in the price of the source yarn.
In the invention, by heavily twisting the polyester multifilament yarn as the warp yarn having very small single-yarn fineness, the twisting operation being not commonly performed in a conventional seat belt manufacturing process the warp yarn can have a strong binding characteristic. Consequently, since the warp yarn for the webbing is relatively bound strongly, without performing a special surface treatment process, abrasion resistance of webbing can be maintained. Obviously, there is no problem if a surface treatment process was also performed.
Further, in the invention, it is possible to roll the cut end portion of the filament which was broken due to a high drawing ratio applied to the filament in the filament drawing process or a high-speed yarn-spinning operation in the yarn spinning process, into among single filament yarns thus twisted, so as to be hidden thereamong, and so that the cut end of the filament can not be remarkably projected from the surface of the webbing.
In such a manner, a drawback in the appearance of a surface of the webbing can be reduced. As the number of twist should be used in the present invention, in order to keep the strength retention at around 80% or higher, even after an abrasion test with a hexagonal bar according to the JIS standard had been completed and which is generally used as a reference, the twist coefficient xcex1 obtained by the following equation (1) has to be 4000 or higher.
xcex1=Txc3x97(D1)1/2xe2x80x83xe2x80x83(1) 
(where T denotes the number of twist t/m, and D1 indicates denier of a warp multifilament yarn)
The present invention is further characterized by employing a twisting process means in that very low processing cost for performing a twisting process on a warp yarn is applied thereto, for example, a twisting process in that in performing a warping process or weaving process by using, for example, a xe2x80x9ccreel having a twisting apparatusxe2x80x9d as shown in the Japanese Patent No. 2,514,884 or a xe2x80x9ccreel having a double twisting apparatusxe2x80x9d as shown in the Japanese Patent No. 2,630,567, a process of twisting the warp yarn of a webbing is carried out simultaneously therewith.
By doing this, a yarn twisting process, which is eliminated for reducing the cost, can be employed again as a new embodiment in a webbing manufacturing technique. The flexibility of selecting a single-yarn fineness of a source yarn which can be used or a permissible range of a defect of breakage in a filament which occurs in the yarn making process, can be expanded by several times with reference to those of the conventional ones.
Further, in the present invention, from the viewpoints of the hexagonal bar abrasion characteristic with respect to weft yarns for a webbing in which the above-mentioned warp yarns are used for the warp yarns thereof, stability of configuration for a webbing or the like, a webbing is woven with weft yarns with a weft yarn density so that the relation between a density and a thickness of weft yarns satisfies the following equation (2) (which is defined as a weft yarn coefficient, hereafter)
weft yarn density (weft yarns/3 cm)xc3x97(D2)1/3 greater than 175xe2x80x83xe2x80x83(2) 
(where D2 denotes total deniers of the multifilament yarn and/or a monofilament as weft yarns).
In the invention, when the weft yarn coefficient is 175 or less, the strength retention after abrasion test with a hexagonal bar becomes 80% or less, so that a webbing having poor hexagonal bar abrasion resistance and many numbers of crossing points formed by a weft yarn and a warp yarn in the webbing may be easily deviated, is obtained.
By weaving a webbing by using, as weft yarns, the polyester monofilament yarn and/or the polyester multifilament yarn, the hardness (rigidity) in the width direction of the webbing is adjusted to obtain comfortableness when the user wears the webbing, or roll up characteristic or reversing characteristic of the webbing when the webbing is taken up by a take-up apparatus, can be prevented.
By properly selecting the fineness of the weft yarn in the range where the weft yarn coefficient is satisfied and by changing the thickness of the webbing, the heat resistance of the webbing can be adjusted so as to prevent melting deterioration in the webbing due to high-speed friction between the webbing and a metal fitting for guiding the webbing or between the webbing and a human body at the time of collision.
A polyester multifilament yarn having a single-yarn fineness, a single yarn strength and the number of twist different from those of the polyester multifilament yarn as mentioned above or a multifilament yarn made of a material other than polyester can be used as a part of warp yarns.
Further, by inserting one or a plurality of the warp yarns having a relatively small fineness to both end portions of the webbing, the shape of the end portions of the webbing is made almost round or cylindrical. Thus, the webbing having the ends which are soft can be provided. To identify an external appearance of a webbing, one or a plurality of the above-mentioned warp yarns may be inserted to an optional part of warp yarns. By changing the weaving construction of a part of warp yarns forming a webbing, the portion of the webbing having a different weaving construction from that of the main portion of the webbing can be used as a mark enabling to identify a webbing having a different specifications form standard ones.